Typically, burners such as those used outdoors include a fuel tank (such as a portable fuel bottle) containing fuel, a burner head and a flow rate adjusting valve. Fuel in the tank is fed to the burner head through the flow rate adjusting mechanism and burned. Such burners include the type in which vaporized fuel is taken out of the fuel tank and burned, and the type in which liquid fuel is taken out of the fuel tank, vaporized in a vaporizer, and the thus vaporized fuel is burned.
In order to control the flow rate of liquid fuel or vaporized fuel, a needle valve, a ball valve, or a regulator valve is ordinarily used.
As disclosed in U.S. Pat. No. 4,575,043, needle valves include a valve box, and a needle shaft and a valve seat both mounted in the valve box, the needle shaft being formed with a thread on the outer periphery and having a tapered tip, and the valve seat being formed with an orifice coaxial with the needle shaft. When the needle shaft is rotated about its axis in one or the other direction, the needle shaft is pushed into or pulled out of the orifice. When the needle shaft is pushed into or pulled out of the orifice, the gap between the tapered tip of the needle shaft and the inner wall of the orifice (which corresponds to the degree of opening of the valve) changes, making it possible to adjust the flow rate of fuel.
As disclosed in JP Patent Publication 2010-236686A, ball valves include a valve formed with a hole and having a shaft. By operating the shaft, it is possible to change the relative position between the hole and the gas passage, thereby changing the width of the flow passage corresponding to the degree of overlapping of the hole and the gas passage. It is thus possible to adjust the flow rate of fuel.
JP Patent Publication 6-235466A shows one of the above-described regulator valves which uses a diaphragm. This valve adjusts the flow rate of fuel by changing the pressure on the secondary side (outer side) relative to the pressure on the primary side (inner side).
After the flow rate has been adjusted, the fuel is fed to the burner through a fuel hose or directly from the fuel supply port of the fuel valve.
Especially if a liquid fuel is used as fuel for a burner and the fire power is adjusted by adjusting the flow rate of the liquid fuel, it is necessary to control the flow rate with high accuracy, because liquid fuel is expanded to 200 times or more of its original volume when vaporized, so that a slight change in the flow rate of liquid fuel can lead to a large change in fire power.
While it is necessary to vaporize liquid fuel in the vaporizer, while the vaporizer is not sufficiently heated immediately after the use of the burner, it cannot be vaporized. Thus, a preheating step is ordinarily carried to preheat the vaporizer by burning a small amount of liquid fuel impregnated in e.g. glass wool provided near the vaporizer.
Any of the above-mentioned needle valves, ball valves and regulator valves offers problems in stably adjusting the flow rate.
In a needle valve, the smaller the thread pitch, the shorter the distance by which the thread advances per unit rotational angle, and thus more finely it is possible to adjust the flow rate. But it is practically impossible to infinitely reduce the thread pitch because the smaller the thread pitch, the lower the resistance to e.g. wear. Thus, it is impossible to adjust the flow rate of liquid fuel in a sufficiently fine manner required for liquid fuel.
Thus, in a burner of the type in which liquid fuel is directly supplied from the fuel tank, the flow rate is ordinarily adjusted in two stages, i.e. a stage (1) in which the flow rate of liquid fuel is roughly adjusted using a needle valve at the fuel outlet port of the fuel tank, and a stage (2) in which after the liquid fuel has been vaporized, the flow rate of the vaporized fuel is finely adjusted by a valve for vaporized fuel which is provided near the burner. This complicates the entire device, increases the cost of the device, and makes it more difficult to operate the device.
It is possible to finely adjust the flow rate of fuel by reducing the diameter of the orifice of the fuel passage in a needle valve or a ball valve. But if a liquid fuel such as gasoline for motor vehicles is used, foreign matter such as soot tends to adhere to the inner wall of the orifice, thus clogging the orifice. Since regulator valves are complicated in structure and made up of a large number of parts, they are expensive and less durable.
The preheating step in which liquid fuel impregnated in e.g. glass wool is burned is extremely troublesome for inexperienced users. Soot tends to be produced during this preheating step, which could soil e.g. the burner and cooking utensils placed on the burner. Flames produced during this preheating step are weaker in force than flames produced by the burner and can be shaken by the wind. Thus, on a windy day, it is difficult to direct the flames produced during the preheating step at the burner (especially the vaporizer), thus making it difficult to sufficiently heat the vaporizer.